The art of ink-jet technology is relatively well developed. Commnercial products such as computer printers, graphics plotters, and facsimile machines employ ink-jet technology for producing hard copy. The basics of this technology are disclosed, for example, in various articles in the Hewlett-Packard Journal, Vol. 36, No. 5 (May 1985), Vol 39, No. 4 (August 1988), Vol 39, No. 5 (October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992) and Vol. 45, No. 1 (February 1994) editions, incorporated herein by reference. Ink-jet devices are also described by W. J. Lloyd and H. T. Taub in Output Hardcopy [sic] Devices, chapter 13 (Ed. R. C. Durbeck and S. Sherr, Academic Press, San Diego, 1988).
Generally, in the thermal ink-jet field, an ink-jet pen is provided with a printhead, having an orifice plate in combination with heating elements. Thermal excitation of ink is used to eject droplets through miniature nozzles and orifices, onto a print medium, forming alphanumeric characters or images using dot matrix manipulation. Other types of ink droplet generators, such as the use of piezoelectric transducers, are also known in the art.
The pen may also serve as a reservoir for storing ink and providing appropriate amounts of ink to the printhead during a printing cycle. Ink can be stored in a contained medium, such as a permeable foam material, in a disposable pen (see e.g., U.S. Pat. No. 4,771,295 (Baker et al.), assigned to the common assignee of the present invention and incorporated herein by reference). Or, the pen can be a free-ink type, where the ink is supplied to a printhead mechanism from an on-board reservoir or, if refillable, from a remote ink supply to a relatively permanent printhead mechanism (see e.g., U.S. Pat. No. 4,929,963 (Balazai), assigned to the common assignee of the present invention, incorporated herein by reference).
While such pens provide a reliable and efficient means of "jetting" droplets of ink from the nozzle plate onto the print medium, the printheads generally require a mechanism to prevent the free flow of ink through the orifices when the printhead is not activated. Without this control, ink may leak, or "drool", onto the printing surface or into the hard copy transport and printer mechanism. Such leaking ink may also build up and cake on the printhead itself, impairing proper operation. Complex pen service stations are often provided where pens can be wiped or activated to "spit" away excess ink.
To alleviate this problem, many ink-jet printers supply ink from the reservoir to the printhead at a slight under pressure, also referred to in the art as "back-pressure" or "negative pressure" operation, such as at about minus three (-3) inches Water Column (WC) lower than the ambient atmospheric pressure at the printhead. To be effective, this pen back-pressure must be maintained consistently and predictably within a desired operating range. That is, the pen back-pressure must be large enough to prevent the unwanted free flow of ink through the orifices, yet at the same time, small enough so that the printhead, when activated, can overcome the back-pressure and eject ink droplets in a consistent and predictable manner. This back-pressure will be affected by changes in either or both the ambient atmospheric pressure or the internal pressure. Likewise, temperature variations may cause the ink and air within the ink-jet pen to contract or expand, also affecting the back-pressure. Therefore, these factors must be accounted for and a mechanism should be incorporated into an ink-jet pen to maintain the back-pressure within the predetermined desirable operating range.
In a foam reservoir pen, the capillary action of the foam will generally be sufficient to create the desired back-pressure. In a free-ink reservoir type ink-jet pen, a variable volume, local reservoir is often employed. For example, the reservoir may be of a biased, flexible material which can expand or contract as shown in U.S. Pat. No. 4,500,895 assigned to the assignee of the present invention and incorporated herein by reference. Or, an on-board ink containment chamber may be provided which includes a pressure regulator device as shown in U.S. Pat. No. 4,509,602, assigned to the common assignee of the present invention and incorporated herein by reference. U.S. Pat. No. 4,677,447, assigned to the common assignee of the present invention and incorporated herein by reference, describes the use of a check valve in a printing device with an on-board ink reservoir that maintains a constant pressure difference between the ink reservoir and the ink-jet printhead. U.S. Pat. No. 5,650,811 (Seccombe et al.), assigned to the common assignee of the present invention and incorporated herein by reference, describes a pressure regulator located on-board an ink-jet pen using an off-board ink reservoir.
As the volume of ink within the reservoir varies due to depletion, thermal or ambient pressure variations, and the like, the volume of the local ink containment chamber also varies. The back-pressure range can be affected by the introduction of gases into the free-ink reservoir. For example, air can be sucked up through the orifice plate or out-gassed from the ink composition. As a biasing regulator mechanism is specifically designed to maintain the back-pressure in the printhead mechanism local ink containment chamber within a predetermined range, such unpredictable and thus unaccounted for gases may adversely affect operation. Therefore, these gases must be removed from a free-ink reservoir if the printhead and regulator mechanism does not have enough compliance to prevent the expanding gases from forcing ink out of the orifices during temperature and altitude excursions.
Thus, there is a need for an gas purge and separator mechanism for free-ink ink-jet pen devices.